As shown in FIG. 1, there is known a rear wheel suspension C in an axle suspension system in which a wheel B is mounted on each of a left-right pair of rear axle shafts A which are connected by way of a differential. This rear wheel suspension C is formed as a semi-floating type in which a bearing D is incorporated at one end side of the rear axle shaft A, and an outer ring side D1 in this bearing D is fixed to a cylindrical rear axle housing E so that the rear axle shaft A is rotatably supported by an inner ring side D2 in the bearing D.
In addition, a rotation detecting device for detecting the rotation speed of the wheel is attached to such a bearing which is used for rotatably supporting the wheel. As the rotation detecting device, a hub to which the wheel is mounted and a bearing unit consisting of a bearing including an inner ring side rotatably supporting the hub and an outer ring side fixed to the suspension are used. A magnetic encoder is fixed to the vehicle inner side in the inner ring side of the bearing of this bearing unit. An annular protection plate for covering the magnetic encoder is provided on the vehicle inner side in the outer ring side, and a sensor is disposed so as to oppose the magnetic encoder through the protection plate interposed therebetween. Thus, there is known a configuration wherein the rotation change of the magnetic encoder which is rotated in conjunction with the rotation of the wheel is detected by the sensor (see Japanese Patent Publication No. 2002-333033A).    Patent Document 1: Japanese Patent Publication No. 2002-333033A